The present invention relates to a method and apparatus for recording and/or reproducing a hologram.
Efforts are being made to develop a holographic recording/reproducing apparatus for recording/reproducing data using a hologram.
In the holographic recording/reproducing apparatus, modulated signal light (on which data is superimposed) and non-modulated reference light are produced from laser light and a holographic recording medium is illuminated such that the signal light and the reference light strike the same point on the holographic recording medium. On the holographic recording medium, the signal light and the reference light interfere with each other and, as a result of the interference, a diffraction grating (a hologram) is formed on the holographic recording medium and data is recorded on the holographic recording medium.
After data is recorded on a holographic recording medium, if the holographic recording medium is illuminated with reference light, diffracted light is produced from a diffraction grating formed in the recording operation. The reproduced light includes data that was superimposed on the signal light during the recording process, and thus it is possible to reproduce the data by detecting the reproduced light using a photosensor.
In some holographic recording/reproducing apparatus, a solid laser or a gas laser is used as a laser light source. However, it is impossible to directly modulate such a type of laser. In such a type of holographic recording/reproducing apparatus, it is known to dispose an external modulator such as a mechanical shutter or an AOM (Acoustic Optic Modulator) in the outside of the laser light source whereby to modulate the laser light after the laser light is emitted from the laser light source. However, the use of the external modulator element makes it difficult to achieve a small-sized optical system and results in an increase in cost.
The above problems can be solved by using a laser diode as the laser light source of the holographic recording/reproducing apparatus. When a laser diode is used as the laser light source, it has been proposed to use an external cavity to enhance the coherence of the laser diode (refer to, for example, Japanese Unexamined Patent Application Publication No. 2001-284715, (in particular, paragraphs [0028] to [0031] and FIG. 1)). The use of the laser diode as the laser light source makes it possible to directly modulate the laser, and thus it becomes unnecessary to provide an external modulator for modulating a laser beam emerging from the laser light source.
In the known external-cavity laser, when the current supplied to the laser diode is continuously varied, the wavelength of the laser light abruptly changes (this change is called mode hopping). The mode hopping occurs at rather short intervals of the current supplied to the laser diode. If the current supplied to the laser diode is set to a value near a mode hopping point, the wavelength of laser light becomes unstable, and it becomes difficult to achieve high performance in recording/reproducing data on/from a holographic recording medium.